Progradational patterns at the head of single units of base-of-slope, submarine granular flow deposits (“Conglomérats des Gâs”, Coniacian, SE France)

Abstract

Upper Cretaceous conglomerates, sandstones and calcarenites outcropping in the Glandage syncline (Les Gâs Gorge) belong to two superimposed “turbidite” systems, the first being the shortest (about two hundred metres thick, five kilometres wide, and a mere ten kilometres long). Both onlap a SW–NE oriented palaeoslope created by faulting in the latest Turonian on the southern edge of the Vercors carbonate platform. The first system is Coniacian in age, the second probably Campanian.This study deals only with the first system which comprises, in stratigraphic order, classical sandstone turbidites, clast-supported conglomerates, and coarse-grained, cross-bedded, sandy calcarenites. The transition between the last two lithologies is progressive, allowing the mechanisms, responsible for apparent lateral accretions found pervasively in both the conglomerates and the calcarenites, to be understood. Here, cross-bedding is not the result of changes in the direction of accretion in “lateral bars” or in oblique infillings of sinuous channels, as is often suggested in other similar deep-water carbonate systems. Each event bed comprises a prograding conglomeratic head plug, blanketed in continuous sedimentation by forward spreading bioclastic calcarenites. The superimposition of such mixed beds gives rise to a low-angle cross-stratification, an unusual feature for a system interpreted as a base-of-slope apron. Conglomerates were probably deposited through a granular flow mechanism at a strong hydraulic jump at the base of the slope. Associated calcarenites show supercritical flow features. The undulating transverse architecture of its large head onlapping the paleoslope suggests it is a multi-apex apron. The sandstone-conglomerate-calcarenite succession of the depositional sequence is interpreted as overall transgressive after the late Turonian tectonic phase that led to the exposure of the Vercors carbonate platform updip.